1. Field of the Invention
The invention relates to an aqueous urea solution supply device and an aqueous urea solution supply method for an internal combustion engine.
2. Description of Related Art
Purification method using aqueous urea solution is known as an example of a method for removing NOx that is contained in exhaust gas of an internal combustion engine. In the internal combustion engine that adopts this purification method, the aqueous urea solution is stored in a tank and a urea adding valve that injects the aqueous urea solution is disposed on an exhaust passage. The aqueous urea solution that is stored in the tank is pressure-fed to the urea adding valve by a pump, and the aqueous urea solution is added from the urea adding valve toward the exhaust gas. The aqueous urea solution added to the exhaust gas is hydrolyzed by heat of the exhaust gas, and then ammonia is generated. The generated ammonia is adsorbed onto a NOx removal catalyst disposed in the exhaust passage and is used when the NOx is removed by reduction.
In some cases, the temperature of the aqueous urea solution in the urea adding valve is raised by the temperature of the urea adding valve being raised with the urea adding valve being exposed to the exhaust gas. Then, the urea adding valve might corrode due to the aqueous urea solution with an elevated temperature. Japanese Patent Application Publication No. 2014-222064 discloses a device that performs a cooling addition (protective injection) for adding the aqueous urea solution to the exhaust gas such that the urea adding valve is cooled. When the aqueous urea solution is added from the urea adding valve to the exhaust gas by this cooling addition, aqueous urea solution with a relatively low temperature flows into the urea adding valve from the tank, and thus the urea adding valve is cooled.